Various types of biometric systems are used more and more in order to provide an increased security for accessing an electronic device and at the same time keep the user convenience at an acceptable level. In particular fingerprint sensors have been successfully integrated in such devices, for example, thanks to their small form factor, high performance and user acceptance. Among the various available fingerprint sensing principles (such as capacitive, optical, thermal etc.), capacitive sensing is most commonly used, in particular in applications where size and power consumption are important.
All capacitive fingerprint sensors provide an indicative measure of the capacitance between several sensing elements and a finger placed on the surface of the fingerprint sensor. Acquisition of a fingerprint image is typically performed using a fingerprint sensor comprising a plurality of sensing elements arranged in a two-dimensional manner, and a block based technique may be applied to the fingerprint sensor for acquiring a fingerprint image, where the blocks of sensing elements are sampled sequentially.
One problem for capacitive fingerprint sensors is that the finger conductivity varies strongly with the humidity of the finger. For dry fingers, the result may be that the outer part of the skin (stratum corneum) has higher impedance (lower capacitance) than the sensor dielectric, so that the combined series capacitance is dominated by the finger impedance. This ridge may then seem more like a valley than like a ridge.
For wet fingers, where water or saline (sweat) fill the valleys, the problem is mainly that the saline is even more conductive than the stratum corneum. With the described capacitive measuring technique this gives a high signal for both valleys and ridges, so that it is difficult to separate the two. When converted to a digital image of the finger, the result may be a low-contrast “inverted” image where the well-conducting valley appears as “black” and the slightly less conductive ridges appear as “dark grey”. Such an image does not easily lend itself to software algorithms aimed at fingerprint recognition.
An exemplary implementation for trying to overcome this problem is disclosed in U.S. Pat. No. 6,330,345. U.S. Pat. No. 6,330,345 discloses an image acquisition method according to which a plurality of images are captured using different settings (such as for dry, normal and sweaty fingers). The segments of the plurality of images are stitched for achieving a fingerprint image of the best possible quality. Even though U.S. Pat. No. 6,330,345 introduces an interesting approach to acquiring a fingerprint image, the additional steps of analyzing and stitching segments of the plurality of images will increase necessary image processing, thereby placing additional constrains of the electronic device including the fingerprint sensing system. Thus, there appears to be room for further improvement in regards to the acquisition of a fingerprint image in conditions when the finger is wet (sweaty), for example limiting any additional computational step when acquiring a high quality fingerprint image.